Single sideband modulation depends on the suppression of one of the sidebands. Heretofore it has been appreciated that the degree to which the unwanted sideband may be suppressed is dependent upon the accuracy of the phase shift of the local oscillator signal. The ideal phase shift is 90 degrees. Any deviation from 90 degree phase shift, degrades the suppression of the unwanted sideband. Typically, a phase shift error of less than one degree is desired. When modulators are manufactured using integrated circuits, it is particularly difficult to achieve such accuracy in the local oscillator phase shift at high radio frequencies (RF). Direct modulator architectures, such as that shown in FIG. 1, require that an accurate phase shift be done at high frequencies and are therefore difficult to design and manufacture.
To avoid the problem of achieving highly accurate phase shift, indirect modulation is often used, in which the phase shift circuitry is implemented at a lower intermediate frequency (IF), where it is easier to achieve the desired phase shift accuracy. An example of a typical indirect modulator architecture is shown in FIG. 2. In an indirect modulator, a quadrature modulator is used to generate a single sideband output at the intermediate frequency. An up-conversion mixer is used to mix the IF output with a radio frequency (RF) local oscillator. However the output of the up-conversion is no longer single sideband, as sidebands are produced at both the sum and difference of the IF and RF local oscillator frequencies. One of these sidebands will be selected and the other rejected by use of a filter such as FF2 shown in FIG. 2. The rejection requirements of this filter make it extremely difficult to implement on an integrated circuit, and therefore a filter external to the integrated circuit is typically used. In addition to eliminating the unwanted sideband, filter FF2 is also used to eliminate unwanted spurious signals and wideband noise that may be generated by the modulator. This function is also performed by filter FF1 of the direct modulator.
The mixers employed in the modulators are often considered to be linear multipliers in order to simplify analysis. The mixers can be thought of as having a linear input port where the information signal is applied (for example, I and Q are connected to the linear input port), and a non-linear switching port to which the local oscillator is applied. The non-linear switching action of the mixers creates outputs that are located at the harmonics of the IF oscillator. Some of the higher order harmonics will be comparable in frequency to the desired output and may interfere with the desired output signal. Other IF harmonics may re-mix in such a way that they create interfering spurious signals that can not be eliminated with filter FF2. Filter F1 of FIG. 2 is used to filter out the IF local oscillator harmonics. This helps to improve the spurious signal performance of the modulator. Required filter performance is not difficult to achieve and this filter is typically implemented on the integrated circuit.
To reduce the generation of undesirable spurious signals through the linear path of the modulator, the linearity of this path must be very good. Preceding the output buffers, the direct modulator of FIG. 1, and the indirect modulator of FIG. 2 can be designed so that the non-linearities are acceptably small for comparable signal levels. The largest contributor to non-linearity for each architecture is the output buffer. Using a power series expansion, the output buffer non-linearity can be analyzed as a function of its input voltage V.sub.IN, as: EQU V.sub.OUT =K.sub.0 +K.sub.1 V.sub.IN +K.sub.2 V.sub.IN.sup.2 +K.sub.3 V.sub.IN.sup.3 +K.sub.4 V.sub.IN.sup.4 + . . .
When the coefficients K.sub.2, K.sub.3, K.sub.4, etc., are each zero, the amplifier is considered to be perfectly linear. To improve linearity, it is necessary to reduce the non-linearities added by the higher order terms, K.sub.2 V.sub.IN.sup.2, K.sub.3 V.sub.IN.sup.3, K.sub.4 V.sub.IN.sup.4, etc., either by decreasing the magnitude of the input voltage, V.sub.IN, or by decreasing the magnitude of the non-linearity coefficients, K.sub.2, K.sub.3, K.sub.4, etc. If we assume that output buffer OB1 (FIG. 1) is identical to output buffer OB2 (FIG. 2), and that the power of the desired output sideband should be the same for both the direct and indirect modulator when driving identical loads, we see that the indirect modulator of FIG. 2 will have poorer linearity than the direct modulator because both the desired and undesired sideband are present in OB2 and contribute equally to the output power. Accordingly, to achieve the requisite power for the desired sideband in the indirect modulator of FIG. 2, its V.sub.IN must be larger than the V.sub.IN of the direct modulator of FIG. 1 which produces only one sideband. Since we have assumed the output power in the desired sideband is identical, and that the output buffers are identical, the terms K.sub.2 V.sub.IN.sup.2, K.sub.3 V.sub.IN.sup.3, K.sub.4 V.sub.IN.sup.4, etc., for the indirect modulator must be larger than the corresponding terms of the direct modulator.
In order to achieve linearity with an indirect modulator that is comparable to a direct modulator, it is necessary either to reduce the level of the input signals to the output buffer or to increase the linearity of the output buffer. Decreasing the input level results in a decrease in output level. This is usually made up with an additional amplifier between FF2 and the PA, an approach that requires additional power. Improving the linearity of the output buffer is typically done by increasing the power dissipated in the buffer. For either approach, the net result is that an indirect modulator, such as that shown in FIG. 2, results in a higher net power consumption in the end application.
A second issue to consider in the selection of a modulator architecture is the ease of implementing the frequency synthesizer portion of the local oscillators. It often simplifies the synthesizer design if the transmit frequency is generated as the sum or difference of a variable radio frequency and fixed intermediate frequency, the technique employed in indirect modulation. In a direct modulator, either these signals must be mixed together with a separate mixer, or the required frequency must be generated directly. Either solution adds complexity and cost to a system using a direct modulator.
In summary, the direct modulator is simple and has better linearity for a given power dissipation and output power than an indirect modulator. However, the accuracy required for the direct modulator's phase shifter operating at RF is difficult to achieve in practice, and the generation of the RF local oscillator may complicate the frequency synthesizer design. The indirect modulator is more complex and is not as linear as the direct modulator for a given power dissipation and output power. However, the accuracy required for the phase shifter operating at IF is easier to achieve than the same accuracy at RF as required by the direct modulator. The indirect modulator may also simplify frequency synthesizer design in some cases.
Accordingly, it would be desirable to combine the advantages of both the indirect modulator architecture and the direct modulator architecture while avoiding the disadvantages of these architectures.